CB23/60/61 Ignition Timing.
Believe it or not this is fairly easy. just a little fiddly.
We will confine ourselves to the basic Charade setups, but the principles are still the same in most cars:- a small signal triggers a discharge of 14 kilovolts or so from a coil or charge capacitor through a spark plug to ignite the fuel mixture in the combustion chamber.
The timing of the spark is supposed to coincide with the piston position that gives the most complete burn for the most advantage. On traditional distributor setups the spark timing is affected by three main factors: static advance, vacuum advance and mechanical (centrifugal) advance.
Mysteries Unravelled
Lets discuss a basic setup with a vacuum advance distributor to explain what happens. Cars with ECUs can use a vacuum advance distributor, but generally the spark timing is done by matching rpm, throttle position, temps, knock sensors, etc with prepopulated maps held in memory..
The static timing is the number of crank degrees before the piston reaches top dead centre (TDC) the spark begins to fire. Generally this is around 5° to 10° before top dead centre (BTDC). Because it is firing before TDC it is called advanced timing (spark in advance of TDC). This gives the fuel mixture enough time to propogate a flame that will ideally move evenly as a wave front through the combustion chamber resulting in complete burn and maximum liberated energy about 10° - 15° ish degrees after top dead centre (ATDC). As the engine speeds up the spark needs to occur earlier (advanced) so a centrifugal governor for mechanical advance is employed.
So the static timing is generally the minimum advance of spark. But this is insufficient when running under part throttle and moderate engine rpm because the amount of air and fuel getting into the cyclinders is being choked off by the throttle plates. Because the volume is less, the density of the mixture is less, the volatility less, than ideal and therefore the flame propogation is longer and relying on the static advance alone would result in an incomplete burn, cooler engine, less power and a trickle of water out of you exhaust pipe from condensation. Enter vacuum advance.
For a good start you want just the static advance, after that a little extra advance (eg 6 camshaft °) is welcome to get a more complete burn. At idle its not a big deal, but as soon as the throttle plate cracks open you want that extra advance and the easy way of doing this is with manifold vacuum. The throttle plate when closed, blocks a small port that is piped to the distributor, when it cracks open the port is subject to the vacuum of the intake manifold. In fact it is seeing the pressure drop across the throtlle plate. The vacuum at the distributor causes a diaphragm actuator to rotate the breaker plate 3 camshaft ° or so if it's a dual action jobbie like the CB23 and the timing is now advanced a total of the static and the vacuum advance degrees (e.g. 5° + 22° = 27°). On single action actuators the vacuum advance is about 10 camshaft °. Please note some people just hook the vacuum line up to the intake manifold, but for a daily drive this can cause rough acceleration and poor emmission control.
So you are just cruising with your throttle plate more closed than open. The air through the throttle body is fairly substantial and under these conditions a second port above the throttle plate is being subject to a vacuum both from the intake manifold and the reduction in static pressure because of high velocity pressure. You can look up venturi effects on the net for an explanation of pressure changes. This second port drives the second stage advance of the distributor and adds another 5° or so advance for better burn, so now you have 5° + 6° + 5° = 16° total advance (note: "total advance" is generally regarded as static plus centrifugal). But what happens when you push the accelerator to the floor for acceleration you ask?
Well this is where things change. All of a sudden you have a wide open throttle(s) tons of fuel and air and the last thing you want is it igniting gob fulls of fuel prematurely and trying to force the piston back down where its's coming from. At this stage the vacuum in the manifold goes to atmospheric pressure or more if you have a turbo/super charger, the velocity pressure decreases and the vacuum signals to the distributor are negated forcing the timing to retard back to static advance angle (or less) plus a bit. What is that bit you ask?
Well from idle up to about 3000 rpm some weights in the bottom of the disitributor housing have been trying to to fly out of the casing under centrifgal force. They are constrained by a pivot on one end and some springs on the other. The springs obviously control the amount of movement or deflection by acting against the centrifugal force. As the spring release the weights, those same weights act as a cam to rotate the distributor spindle, effectively advancing the timing. Notches in a plate connected to the spindle dictate how far the rotation will be and some plates have two different sized notches for two stage action or easy range swap over. The centrifugal or mechanical advance therefore provide compensation for the speed of the motor because the flame can only spread so fast as a function of the burn rate of the fuel. The number of maximum degrees and the rate of change varies from motor to motor, but it is generally around 50° max.
So a CB23 engine cruising around 3000 rpm could have a total advance of 5° + 22 °+ 21 ° = 48 crank ° and a CB60 about 10° + 26° - 16° = 20 crank ° max (no vacuum advance, but 16° retard at 3000rpm with boost present). Note that when you look at the notch on the centrifugal advance arm and see a number stamped,you multiply this number by two to get crank degrees. Only the notch that has the mechanical stop is functional.
So the static advance is set by you when you use the timing light, the vacuum advance rotates the breaker plate in reponse to a vacuum actuator and the centrifugal advance rotates the rotor spindle in response to engine rpm.
Getting Down and Dirty
This engine has 5° static advance, 22° vacuum advance and 21° centrifugal advance. It has a two stage vacuum advance diaphram attached to the dizzy.The outer bellows requires a relatively strong vacuum to advance while the inner retracts the arm with minor vacuum available.The outer is connected to either a permanent vacuum port on the manifold spacer or an alternative port on the spacer that opens on throttle opening. The inner is connected to the AD port that goes into vacuum when a strong air flow through the primary barrel is present (e.g. about 1600 rpm).
If you are just putting the distributor in you will need to find TDC of the engine so you can, spark at the right time and align the rotor button electrode to the correct electrode on the distributor cap. Now you can do this two ways:- the methodical approach or the trail and error approach. Both ways work so lets explain both:
As you know the camshaft that drives the dizzy (distributor) revolves at half the speed of the crankshaft. In engines just about everthing concerning degrees is in relation to to crankshaft degrees, lobe separation angles of the camshaft being one of the few exceptions. So when we say we are talking degrees here we are talking crankshaft degrees and we all know that a four stroke engine has two complete revolutions per cycle or 720° while the camshaft revolves only 360° per cycle. This is important to know when looking at the position of the rotor.
Number one cylinder (furthest away form the gearbox) is the index pot for all the timing and alignment marks on the engine So we need to find the compression stroke out of the four strokes for number one pot.. If we just looked at a mark on the flywheel we can't tell because it could be either the compression stroke or the exhaust stroke
Methodical approach:-
If you have just done a head rebuild no doubt you have just set the valve lash and in the process rotated the engine about 180° to set intake 2 and exhaust 3 rockers. So now you are going to have to rotate a full revolution plus the remaining 180° to get back to around where the compression stroke ends. This means you will rotate clockwise through one timing mark on the flywheel.
If you don't know where you are currently at you will need to find where TDC of the compression stroke. So you will need to take the rocker cover off. Remember what we said about the camshaft rotation? Well we are going to use it now.
With the rockers exposed to view you will notice theres always at least two of the six rockers that have a bit of movement or lash because their cam follower pads are near the heal of the cam lobe . We are going to use this as we know when a mated pair of rockers are slack the associated cylinder is at the top of it's compression stroke. So all you have to do is DISCONNECT THE BATTERY -VE and use a spanner on the bottom belt pulley bolt head to rotate the the crakshaft until No1 rockers are no longer "rocking" and feel slack. At the same time you will notice intake No3 rocker and exhaust No2 rockers will also be slack. This has got you into the ballpark of where the TDC is.
Now look down to an opening on the bell housing where the gearbox meets the engine and you will see a small square portal to the flywheel. On the flywheel is a fairly obvious dimple, this is the timing mark. When it lines up with the notch in the plate you are at the static timing advance position. You may have to rotate the motor back or forth to find the dimple. Get some whiteout and put a dollop in the dimple so it is easy to see later on.
So now you put your dizzy in, which can take a few trys in getting alignment correct. First thing to do is to make a mark on the alloy housing in line with number 1 dizzy cap electrode. This is the electrode that is just anticlockwise to the retaining bracket and sits about 10 o'clock position normally. Take the cap off and rest it somewhere safe. You know the dizzy must have the retaining bracket slot over the retaining screw hole so that is your first constraint. You need to juggle the distributor into position so that you have the retaining bracket slot over the screw hole, the rotor button arm pointing at the mark you made for No1 electrode and the two cap clips free of obstucting cables.
Put the retaining screw in and semi tighten it enough that you can just rotate the dizzy by hand.
The next step is to get a bit more precise with the positioning. If you have an ohmmeter, grab it. If not get your glasses. Keeping an eye on the points, rotate the dizzy and make sure the points gap is about the thickness of a hacksaw blade maximum and that they close fully when on the flat of the rotor cam. Now what we are looking for is the position where the points just open. This is easy with an ohmmeter, you just put one probe to ground and the other to the terminal of the wire that goes to the coil and look for a high resistance reading. If you are lucky enough to have the electronic version you just line up the tooth on the reluctor rotor wheel with the centre of the coil pickup pole.
Now all you have to do is put the cap back on connect everything else up , block the vacuum hoses at the dizzy with a screw and start the engine. Once started you clamp a good timing light to No1 lead, point it at the newly whitened dimple on the fly wheel and carefully rotate the dizzy until the dimple is aligned with the notch. Turn off engine and tighten the dizzy retaining screw.
Restart engine and point timing light at dimple, rev engine ( by rotating the throttle arm) gradually to around 3000 rpm and look for the dimple to progressively move (advance) under the influence of the centrifugal weights. Let the engine return to idle and put the outer vacuum hose on the dizzy. Depending on the setup it may immediately advance the timing 12° or so, if not it should advance once you start to open the throttle. Now take the plug out of the other vacuum advance hose that goes to the AD port of the carby and you should notice no suction at the open end with your thumb. Rev the engine to about 1600 rpm and you should notice a vacuum. Connect the hose to the inner bellows port of the dizzy.
Job done.
There are individual vacuum diagrams in the engine manual section 12. An alternative for the Cb23 that seems to work well is this
Trial and Error approach:-
This is a lazy method, and is the same as described above, except rather than lifting the rocker cover you just line up the timing dimple on the flywheel, put the dizzy in and align it, try to start the engine, if it backfires through the carby without starting it's probably out by 360°. Simply pull dizzy out and rotate 180°, put back in and do final adjustments.
This engine has 10° static advance, 20° vacuum advance and 27° centrifugal advance and 15/16° boost retard without vacuum advance (there is unlikely to be any vacuum advance when the centrifugal advance is engaged due to boost). It has a single stage vacuum advance/ boost retard diaphragm attached to the dizzy. See graph
The procedure is the same as for the CB23, except there is only one hose to the dizzy (from the AD port). The suction on this hose will occur as the engine revs around the 1600 rpm mark.
Note for both engines the ignition sequence is 1-2-3. The dizzy rotor rotates clockwise. No1 position =10 o'clock. No2 =2 o'clock , No3 =6 o'clock.
Picture of CB60 timing dimple in timing notch. White line is TDC.
This is a reluctor/coil type distribitor that can be retrofitted easily into the G11 and G100 series. Setting initial timing is the same as previously explained, except all you have to do for rough setting is line up the reluctor spoke for number 1 cylinder to the coil pickup pole. More info see here
Dwell
Just a comment about dwell. This is generally the time the coil is charging (points closed). It is usually measured in degrees of cam rotation, but crankshaft degrees are also used just to complicate matters. There are several rules of thumb calculating dwell, for example (( 360°/cylinders) x 0.5 (or 0.75 etc). On the CB 23/60 and HCE the angle is actually about 62° +- 4° :- 3 & 4 cylinder engines have way more time on their hands than V8's at the same revs so can get away with big a dwell angles..
Ideally the dwell should be just sufficient to charge the coil at peak RPM. In practice there is a marked degradation in spark at high revs because the coil with a static dwell angle duration can't recover in time: the dwell has been preset to provide the best average for low and high revs. A large dwell at low rpm will result in prolonged and higher peak primary currents and tend to overheat the windings.
In an ideal world a points type coil takes about 80 microseconds for coil saturation, a TI coil about 125 microseconds and CDI about 6 microseconds rise time. If we take our CB23 at 6500 rpm, it has 18.46 milliseconds to get three sparks in or 6 millisec/cylinder. Of that about half is taken up in dwell time, so 3 millisecs to get the magnetic flux build up. Ideally the coils should cope easily, but if you consider points burn, bounce, their arms distort and they have an arc suppressing capacitor in parallel, things start to go awry. Add in capacitive reactance and heating of the coil itself, centrifugal forces, and normal wear and tear and you can inderstand why solid state systems have become popular. Throw it all into an eight cylinder and the problems compound markedly.
Long dwell angles can be detrimental to coils,but high reving engines need a large enough time to charge the coil. To get around long dwell durations at low revs, conventional can type coils are wound for 6 volt operation and a ballast resistor added in series with the primary winding. Transistor Ignition (TI) coils don't need to be derated as the ignitor regulates the required dwell time.
And remember for every one degree (1°) increase in dwell your igniton timing will retard two degrees (2°). Likewise if you decrease dwell your timing will advance.
And Just One More
The next time you ponder crankshaft sensors versus camshaft sensors for ignition: -by now you should able figure out that at high revs you camshaft timing belt will stretch and actually retard your ignition and valve timing. Just though I would throw that in as food for thought
Want to know more about ignition timing? Click here
